SQL order by with group by query - sql

Question

I have an EMPLOYEES table :
employee_id(1) hire_date(15-2-2001)
employee_id(2) hire_date(2-2-1999)
employee_id(3) hire_date(11-2-2003)
employee_id(4) hire_date(6-7-2001)
I want to display the YEAR with the highest number of employees hired in, with the number of employees hired each month.
I tried this :
select extract (year from hire_date)
from employees
where max(count(employee_id))=count(employee_id)
order by extract (year from hire_date);
and I keep getting an "ORA-00934: group function is not allowed here"
What am I doing wrong?
I'm using ORACLE 10g Express.

Answer 1

The idea is that you can use aggregation and window functions to get the totals by month and year. Then you can choose the largest using row_number() or dense_rank().
select ym.*
from (select ym.*, dense_rank() over (order by year_cnt, year) as seqnum
from (select extract(year from hire_date) as yyyy,
extract(month from hire_date) as mm,
count(*) as cnt,
sum(count(*)) over (partition by extract(year from hire_date)) as year_cnt
from employees
group by extract(year from hire_date),
extract(month from hire_date)
) ym
) ym
where seqnum = 1
order by yyyy, mm;
Hmmmm, you can do this without so many subqueries:
with ym as (
select extract(year from hire_date) as yyyy
extract(month from hire_date) as mm,
count(*) as cnt,
sum(count(*)) over (partition by extract(year from hire_date)) as yearcnt
from employees
group by extract(year from hire_date), extract(month from hire_date)
)
select *
from ym
where yearcnt = (select max(yearcnt) from ym)
order by yyyy, mm;
Of course, this returns multiple years if two years have the same maximum value.

Answer 2

With PL/SQL I found this :
declare
recuperation float;
CURSOR newRequest(recuperationDonnes float) is select count(employee_id) as nombreEmployes,
extract(month from hire_date) as mois from employees where
extract(year from hire_date) = (recuperationDonnes) group by extract(month from hire_date);
a float;
a1 float;
begin
select extract (year from hire_date) as annee into recuperation from employees having count
(employee_id) >= all (select count (employee_id) as emp from employees group by extract(year
from hire_date)) group by extract(year from hire_date);
OPEN newRequest(recuperation);
LOOP
FETCH newRequest into a,a1;
Exit when newRequest%NotFound;
dbms_output.put_Line('Year: '||recuperation||' mois: '||a1||' nombreEmployes: '||a);
END LOOP ;
CLOSE newRequest;
end;